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On 29 December 2013, ESA’s Mars Express will make the closest flyby yet of the Red Planet’s moon Phobos, skimming past only 45 km above its surface.

As the spacecraft passes close to Phobos, it will be pulled slightly off course by the moon’s gravity, by a few tens of centimetres. This small deviation will be measured using the spacecraft’s radio signals, and then translated into measurements of gravity, mass and density at different locations on the moon.